1. Field of the Invention
The present invention relates to a reference clock signal generator for generating a reference clock signal for recording information on a disk in a sampled servo format or reproducing information recorded therein.
2. Description of the Related Art
A sampled servo type optical disk will be explained at first with reference to FIG. 1. In the figure, a substrate 1001 of the optical disk is formed by a resin such as polycarbonate having a thickness of 1.2 mm for example. On one surface thereof, clock marks (also called as a clock pit) 1005, first and second wobble marks 1006 and 1007 among marks called wobble marks (also called as a tracking mark or tracking pit) are formed by such a method as injection. The clock marks 1005 and wobble marks 1006 and 1007 are located on and near intersections of radial straight lines originated from a center O of the substrate of the recording medium 1001 and center lines of spiral or cocentric tracks shown by dashed lines. The first wobble mark 1006 and second wobble mark 1007 for tracking servo are disposed at positions slightly shifted (e.g. 1/4 track pitch each) on the both sides of the center line of the track before and after the clock mark 1005 for synchronization. The clock mark 1005 and wobble marks 1006 and 1007 form a servo area 1002. An information area 1003 is formed also radially between each servo area 1002. Marks 1011 are disposed on intersections of a radial straight line 1013 originated from the center O of the substrate of the recording medium 1001 and the center lines of the spiral or cocentric tracks shown by the dotted lines 1004 in several information areas 1010 among a plurality of information areas on one track. Further, marks 1012 are disposed on intersections of a radial straight line 1014 originated from the center O of the substrate of the recording medium 1001 and the center lines of the spiral or cocentric tracks shown by the dotted lines 1004. The marks 1011 and 1012 are formed by means of injection or the like similarly to the clock marks 1005. Marks indicating an address of the track are formed in the information area 1010 and marks indicating data are formed in an information area 1003 similarly by means of injection method or the like. A reflecting film made of aluminum or the like is formed on the surface thereof. Here, an angle formed between the straight lines 1013 and 1014 is kept the same in every information areas 1010. Further, the angle is set at a value which is not same as the angles formed by these radial lines which are formed by the each marks in the servo areas 1002, information areas 1003 and information areas 1010. Accordingly, when the disk is rotated at a constant number of rotations, the time interval from the mark 1011 to the mark 1012 does not exist in other areas. The time interval from the mark 1011 to the mark 1012 is generally called as an unique distance, which shall be referred to as "UD" hereinafter. A typical diagram (a) shown in FIG. 2 shows the arrangement of the marks (pits) on the disk shown in FIG. 1. The first wobble marks are shown by 1006a through 1006d, clock marks are shown by 1005a through 1005d and second wobble marks are shown by 1007a through 1007d. By the way, the marks (not shown) representing the address of the track are formed in the address area of the information area 1010 and the marks (not shown) representing data are formed in the information area 1003. All of the marks are formed in synchronization at positions determined by equally dividing intervals between the clock marks by a predetermined value. Accordingly, when information is to be reproduced or recorded, a reference clock signal which becomes reference for the reproduction or recording of the information is produced by means of a PLL (Phase Locked Loop) circuit based on clock mark signals obtained by detecting the clock marks. A waveform (b) shows the reference clock signal. As described above, the period of the UD area from the mark 1011 to the mark 1012 is not generated in other areas. Accordingly, the clock marks 1005 disposed at a predetermined interval from the mark 1012 are sampled out by detecting this UD areas. In the same time, the aforementioned reference clock signal is generated based on the detected clock marks 1005, using the PLL circuit. The clock marks 1005 disposed at a predetermined period based on the UD area are detected before when an operation of the PLL circuit become stable. When the operation of the PLL circuit becomes stable, gate signals for detecting clock marks are produced based on the reference clock signal generated by the PLL circuit to detect the clock marks 1005. Further, based on the reference clock signal, gate signals for detecting the first wobble marks 1006 and second wobble marks 1007 are produced. A difference of reflected light amount from the disk caused by the first wobble mark and second wobble mark is detected using the gate signals to detect a deviation of a light beam from the center of the track for tracking control.
As described above, if the number of the UD areas in one track is reduced in the prior art optical disk drive apparatus to enhance recording density, a number of the gates for detecting clock marks produced based on one UD increases. Generally, the position of the clock mark fluctuates due to decentering of the disk or fluctuation of rotation of a motor. Accordingly, the gates for detecting clock marks may shift from the positions of the clock mark as time elapses from a point of time when the UD area is detected. However, because several tens of clock marks have to be accurately detected in order for the PLL circuit to become steady, the PLL circuit cannot be put into a stable state.
Further, in the prior art optical disk unit, pulse signals which correspond to marks in a period when the gate signal for detecting clock mark is opened are input to the PLL circuit to generate the reference clock signal. In this method however, if pseudo pulses are generated due to noise and others in the period when the gate signal for detecting clock mark is opened, the PLL circuit operates based on the pseudo pulses. As a result, the PLL circuit outputs signals different from the reference clock signal. Because the gate signal for detecting clock mark is produced based on the reference clock signal which is an output signal of the PLL circuit, the clock marks cannot be detected. As a result, the output signal from the PLL circuit further shifts from the reference clock signal.
Further, because the gate signals for detecting wobble marks also shift in the same manner, the deviation of the light beam from the center of the track cannot be detected, making the tracking control unstable.